This application incorporates by reference Taiwanese application Serial No. 90102298, Filed Feb. 2, 2001.
1. Field of the Invention
The invention relates in general to a scanner and a method for merging images, and in particular, to a scanner with a reference pattern for merging images and a method for the same.
2. Description of the Related Art
As it is well known in the art, optical scanners produce machine-readable data which are representative of the scanned object, e.g. a page of printed text. Most optical scanners employ line-focus systems in which light from an illuminated scan line on the object is imaged by a lens onto a linear photosensor array or detector positioned remotely from the object. The linear photosensor array is typically a single dimension array of photoelements that correspond to small area locations along the illuminated scan line. These small area locations are commonly referred to as “picture elements” or “pixels.” Each photoelement produces a data signal that is representative of the intensity of the light from the corresponding pixel. The data signals from the photoelements are received and processed by an appropriate data processing system which may subsequently store the data on a suitable medium or generate a display signal, from which an image of the object is reproduced using a display device such as a CRT or a printer.
Scanners with high resolution need to be accompanied with high-capacity memories because images with higher resolution require more memory space. However, when the issue of cost is considered, increasing the memory is not a cost effective solution. Instead, multi-step scanning is used to solve the problem of inadequate memory. That is, a whole image can be divided into smaller pieces and each partial image is scanned at one time. In multi-step scanning, the issue of precisely merging each partial image soon arises.
Conventionally, if the memory is inadequate, the carriage slows down until it stops completely, and where the carriage is located is called the image scan line. The carriage is driven to move back to the image merging scan line, and then continues the scanning process. The carriage is driven by a step motor, and the slow-down and speed-up motion of the carriage is usually unstable and even shakes seriously. As a result, the images captured as the carriage slows down and speeds up are not clear.
FIG. 1 shows a conventional flat bed scanner 100 with a multi-step scanning function. The scanner 100 includes a carriage 102, which scans a document 106 lying on the scan area 104. If the memory (not shown) of the scanner 100 is inadequate to store the image data, the carriage 102 is positioned at one of the scan lines, which is called the image merging scan line L1. The process of driving the carriage 102 back to the image merging scan line L1 is described as follows. First, the carriage 102 slows down from the image merging scan line L1 and moves along the scan direction for a distance W, from left to right as the arrow shown in FIG.1. Eventually, the carriage 102 stops at a stopping scan line L2. The distance W is controlled by a step motor (not shown) and thus can be transformed into Y steps. Please note that the slow-down motion of the carriage 102 is unstable and can even shake. Next, the carriage 102 is driven for Y steps from the stopping scan line L2 to the image merging scan line L1 along the direction shown by the arrow in FIG. 1 from right to left. During the motion of returning to the image merging scan line Li, the carriage may also shake.
Furthermore, the tension of the transmission belt used for driving the carriage 102 during forward and backward motion is not equal. As a result, the carriage still cannot reach the image merging scan line L1 precisely, thus producing an unclear image, even though the step motor drives the carriage 102 back for Y steps.
A method is proposed to overcome the aforementioned error. By comparing the image similarity, it is ensured that the carriage 102 returns back precisely to the image merging scan line L1, which is described as follows. The image of the document 106, which the carriage 102 captures at the image merging scan line, is defined as a standard image. The carriage 102 will not continue to capture images until an image with high similarity as the standard image is found during its forward motion after the backward motion. However, images with high similarity can occur in several parts in a single document, such as a blank space. This could result in mis-recognition of the standard image and further problems such as image overlapping and gaps.